Albert Barnes Commentary 1 Chronicles 5:17

"All these were reckoned by genealogies in the days of Jotham king of Judah, and in the days of Jeroboam king of Israel." — 1 Chronicles 5:17 (ASV)

The writer here refers to two registrations: one made under the authority of Jeroboam II when he was king and Israel was flourishing, and the other made under the authority of Jotham, king of Judah, during the time of trouble that followed the great invasion of Tiglath-pileser. It is not surprising that a king of Judah exercised some form of lordship over the trans-Jordanic territory during this period.